In her first diplomatic visit to the United States, British Prime Minister Theresa May applauded President Donald Trump's new approach to trade and foreign policy, but also warned against trusting Russia and abandoning long held alliances.

May's plaudits to Congressional Republicans gathered for a retreat in Philadelphia came on the same day as Trump's administration set off a diplomatic spat with Mexico. The president's stated plans to build a wall on the U.S.-Mexican border and to possibly levy a 20 percent import tax on the country's goods caused Mexico's president to cancel a planned White House visit and meeting with Trump.

In the speech, May also touted Britain's “special relationship” with the U.S. and all the fruit it has borne. The United Nations, the World Bank, International Monetary Fund and NATO, she said, were all products of the countries' long friendship and desire to build world peace — an ideal that stretches back to before the First World War.

“Some of these organizations are in need of reform and renewal to make them relevant to our needs today,” May said. “But we should be proud of the role our two nations — working in partnership — played in bringing them into being, and in bringing peace and prosperity to billions of people as a result.”

She added that it was unacceptable and unfair for the U.S. to take the brunt of the security concerns on behalf of the world — a nod to concerns Trump raised on the campaign trail — but plainly stated that Britain has always met its defense commitments.

May detailed how much the U.K. spends on defense and overseas development as well as where the country has stationed troops.

Throughout the speech, the British Prime Minister often turned to the example of President Ronald Reagan and former Prime Minister Margaret Thatcher who worked together through the end of the Cold War. Their actions, she said, would be helpful when considering a relationship with Russia.

“When it comes to Russia, as so often it is wise to turn to the example of President Reagan who — during his negotiations with his opposite number Mikhail Gorbachev — used to abide by the adage ‘trust but verify.’ With President Putin, my advice is to ‘engage but beware,’” May said.

May then doubled down on Britain’s support to defeat ISIS. She urged focus on stopping radicalization; called the Iran nuclear deal “vitally important for regional security”; discussed Brexit and Britain’s continued bond with the European Union; and she emphasized the importance of the U.S.-U.K. trade relationship.

“As we renew the promise of our nations to make them stronger at home,” she said, “in the words of President Reagan as the ‘sleeping giant stirs’ — so let us renew the relationship that can lead the world towards the promise of freedom and prosperity marked out in parchment by those ordinary citizens 240 years ago.”

May is scheduled to pay a visit to Arlington National Cemetery Friday morning, followed by a meeting with Trump at the White House in the afternoon.

Phil McCausland

Phil McCausland is an NBC News reporter focused on the rural-urban divide.